1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus that transmits ultrasonic waves to a patient, receives the waves reflected from the patient and processes the waveforms received, thereby displaying, on a screen, tomograms for use in making a diagnosis. More particularly, this invention relates to a two-dimensional ultrasonic probe to be connected to an ultrasonic diagnostic apparatus that can provide three-dimensional images in real time, and also to an ultrasonic diagnostic system.
2. Description of the Related Art
Any ultrasonic probe comprises a two-dimensional array transducer that has elements arranged in the form of a two-dimensional lattice.
A conventional two-dimensional array transducer comprises a backing material and a plurality of ultrasonic transducer elements. The transducer elements 6 are arranged on the backing material, in the form of a two-dimensional lattice. Two electrodes are provided for each transducer element. One electrode is provided on the acoustic emission surface of the element, and the other electrode is provided on the back of the element, which contacts backing material. These electrodes are connected to transmitting circuits (not shown) and receiving circuits (not shown). Further, an acoustic member, such as an acoustic adjustment layer, an acoustic lens or a bio-contact member, is arranged on the acoustic emission surface of each transducer element.
FIG. 1 is a block diagram presenting the configuration of a conventional ultrasonic diagnostic system.
As FIG. 1 shows, the ultrasonic diagnostic system comprises an ultrasonic probe 10a and an ultrasonic diagnostic apparatus 20. The ultrasonic probe 10a has a two-dimensional array transducer 2, a transmitter/receiver disconnecting circuit 12, a transmitting circuit 14, a receiving circuit 16, and a connector 18. The ultrasonic diagnostic apparatus 20 has a control circuit 22, a signal-processing circuit 24, and a display 26.
The two-dimensional array transducer 2 has transducer elements, each connected to a signal line. The signal line is connected to the transmitting circuit (balser) 14 and the receiving circuit (receiver) 16, both provided in the ultrasonic probe la or in the ultrasonic diagnostic apparatus 20. (In the case shown in FIG. 1, the transmitting circuit 14 and the receiving circuit 16 are provided in the ultrasonic probe 10a.) In the ultrasonic diagnostic apparatus 20, the signal-processing circuit 24 performs analog-to-digital conversion on the signal that the receiver 16 has received, so that a tomogram may be displayed on the display 26 (e.g., CRT monitor) after an envelope, for example, has been detected. Further, since the two-dimensional array transducer can transmit and receive ultrasonic waves coming in any directions in space, the signal can be converted to data representing a tomogram of any desired region or can be subjected to three-dimensional rendering. Hence, the display 26 can display a tomogram or a three-dimensional image in real time.
In the conventional one-dimensional array transducer, the strip-shaped transducer elements are linearly arranged. About 100 elements are so arranged in most cases. By contrast, in any two-dimensional array transducer, thousands of transducer elements are arranged in rows and columns, and the probe cable is thick if it contains the signal lines of all transducer elements. The thicker the probe cable, the lower the operability of the ultrasonic probe having the two-dimensional array transducer. In view of this, most ultrasonic probes incorporate transmitting circuits and receiving circuits.
Two electrodes are provided, respectively, on the acoustic emission surface and back of each element of a two-dimensional array transducer. In most two-dimensional array transducers, the electrodes provided on the acoustic emission surfaces are bundled together and connected to the transmitting circuit and receiving circuit through a transmitter/receiver disconnecting circuit, whereas the electrodes provided on the backs are connected, independently of one another, to the transmitting circuit and receiving circuit through the transmitter/receiver disconnecting circuit. In this case, the voltage of the pulses transmitted is generally 100V or more. This voltage raises breakdown problems in most ICs manufactured by the ordinary process of producing low-breakdown-voltage devices.
Therefore, the transmitter/receiver disconnecting circuit is constituted by an IC manufactured by a special process of producing high breakdown-voltage devices. The transmitter/receiver disconnecting circuit is inevitably not only expensive, but is also large and consumes much power. If incorporated into a probe, the probe will be large and have low operability. Further, the transmission voltage must be reduced to keep the probe temperature below a prescribed value, thereby ensuring safety. If the transmission voltage is so reduced, the sensitivity of the probe will decrease. Consequently, the probe will raise problems in terms of image quality.
In view of the above, the transmitting circuit 14 and the receiving circuit 16 may be connected, respectively, to the electrodes 2a provided on the acoustic emission surface of the transducer 2 and be connected to the electrodes 2a provided on the back of the transducer 2, as is shown in FIG. 2. In this case, the receiving circuit 16 is held short-circuited while the transmitting circuit 14 is transmitting a signal, and transmitting circuit 14 is held AC short-circuited, while the receiving circuit 16 is receiving a signal. The transmitting circuit 14 and the receiving circuit 16 are thus disconnected from each other. Such an ultrasonic probe as shown in FIG. 2 is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-41730. In this technique, the transducer can, by itself, disconnect the transmitting circuit and the receiving circuit from each other. A transmitter/receiver disconnecting circuit need not be used at all. The receiving circuit can be an inexpensive IC manufactured by the ordinary process of producing low breakdown-voltage devices. In addition, since no transmitter/receiver disconnecting circuit is required, the ultrasonic probe can be smaller and consumes less power.
Even if the technique disclosed in the Jpn. Pat. Appln. KOKAI Publication No. 2004-41730 is employed, however, transmitting circuits and receiving circuits must be provided in the same numbers as the transducer elements. In order to prevent an increase in the size of the circuitry provided in the probe, the total number of elements should be smaller than a certain value.
Generally, it is necessary to raise the frequency or increase the aperture in order to attain a high resolution. If the frequency is raised or the aperture is increased, while using a limited number of transducer elements, however, the product of the frequency and the pitch of elements will inevitably increase. Consequently, the grating lobe, i.e., transmission or reception in a direction other than the intended direction, become prominent. Hence, the frequency cannot be raised or the aperture cannot be increased in order to attain a high resolution.
In order to attain a large aperture, transmitting circuits and receiving circuits may be used in smaller numbers and a limited number of channels may be connected to these transmitting and receiving circuits. This technique (known as sparse arraying) decreases the ratio of the effective transmission-reception area to the aperture area, reducing the sensitivity of the probe or generates side lobes in the same way as grating lobes are generated. The side lobes decrease the resolution, because they extend in various directions.